Conventionally, there is known, as a cooling structure for a multi-cylinder engine, a structure in which a water jacket is formed in a cylinder block in such a manner as to surround a plurality of cylinders, and a pressurized cooling liquid is introduced from a water pump to the water jacket for cooling an engine.
Further, for the purpose of improving the cooling performance, as disclosed in Japanese Patent No. 3,596,438, disposing a spacer member for separating the inner space of a water jacket within the water jacket is proposed. Specifically, Japanese Patent No. 3,596,438 discloses a configuration, in which a cylinder block includes a water jacket, an introducing portion for introducing a pressurized cooling liquid fed from a water pump to the water jacket, and a discharging portion for discharging the cooling liquid from the water jacket, and a spacer member with a lower portion thereof having a smaller thermal conductivity than an upper portion thereof is disposed within the water jacket. According to this configuration, it is possible to increase the cooling performance with respect to an upper wall of a cylinder bore wall, whose temperature tends to be high as compared with a lower wall of the cylinder bore wall, because the upper wall is close to a combustion chamber as compared with the lower wall. This makes it possible to suppress a temperature difference between portions of the cylinder bore wall in the axial direction thereof, and to suppress non-uniform deformation of the cylinder bore wall.
However, Japanese Patent No. 3,596,438 fails to disclose a configuration for suppressing a temperature difference between an intake side portion of a cylinder block, and an exhaust side portion of the cylinder block, whose temperature tends to be high as compared with the intake side portion due to heat transferred from exhaust gas. As a result, the cylinder bore wall may be non-uniformly deformed due to a temperature difference between the intake side portion and the exhaust side portion of the cylinder block, and fuel economy may be deteriorated as a result of an increase in sliding resistance of pistons.